Goblet with Guinness head on bar (5)
I believe the answer is:
grail
'goblet' is the definition.
(grail is a kind of goblet)
'guinness head on bar' is the wordplay.
'head' indicates taking the first letters.
'on' is a charade indicator (letters next to each other).
'bar' becomes 'rail' (rail is a kind of bar**).
The initial letter of 'guinness' is 'g'.
'g'+'rail'='GRAIL'
'with' is the link.
(Other definitions for grail that I've seen before include "what some look for" , "Last Supper cup" , "Holy object of search" , "Round Table Knights sought the Holy ....." , "Legendary cup sought by knights" .)